This invention relates to methods and apparatus for measuring tension in stressed cables and in particular to methods and apparatus which utilizes the relationship between the natural transverse vibrations in the cable and the tension in the cable.
It is often necessary to ascertain the tension value of stressed cables in a quick, reliable manner without permanent attachment of a measuring device thereto. In applications involving guy wires, mast supports, tension structures etc., measurement of cable tension becomes necessary in order to adjust stressed members for desired values and for balancing interacting elements thereof. Alternatively, tension measurements are necessary for the apprisal of excessive cable stresses thereon.
A known method for determining tension in a given length of stressed cable incorporates the knowledge of the natural frequency of vibration of the length of cable. The relationship between the tension and the fundamental frequency of transverse vibration is expressed by: EQU F=(1/2L).sqroot.S/r, or EQU S=(2 LF).sup.2 r
Where
S=tension in the cable PA0 L=length of unsupported cable free to vibrate PA0 F=fundamental frequency of vibration of the length of cable. PA0 R=linear mass density of the cable
Prior art tension measuring devices such as U.S. Pat. No. 3,540,271 determine tension in stressed cables by ascertaining the value of the fundamental frequency of vibration thereof and utilizing this value to represent cable tension. Let it be noted from the established tension-frequency formula that tension in a stressed cable is proportional to the square of the vibration frequency and the square of the length of cable being measured. Such prior art devices however, obtain tension readings indirectly by displaying a signal proportional to the cable frequency on an indicating device such as a meter, the scale of which is calibrated in proportion to the square root of tension in order to account for the nonlinear relationship between tension and frequency. Readings calibrated in this manner however result in nonuniform resolution within the range of the scale, indicia being more densely distributed at the higher end than at the lower end thereof, which significantly limits the the accuracy and the ease of readability of such devices.
Another prior art device converts a signal which is proportional to frequency to one which is proportional to a frequency squared signal by employing a conventional squaring circuit. Most squaring circuits however are very sensitive, relatively expensive, and involve complex circuitry.
Prior art tension meters have also included devices to sense the vibrational motion of the cable which require accurate positioning with respect to the cable because of the weak signals generated therefrom. Such devices are often fixed in a permanent position of close proximity to the cable being measured.
Other cable tension measuring devices have included relatively complex mechanisms for exciting the cable into its natural oscillations. Such prior art devices include magnetic type excitation and sensing mechanisms which require the cable to be composed of a metal and often a ferrous material. Such devices have been bulky, cumbersome, expensive, and require complex electronics to accomplish the measurement.
The term "cable" hereinafter comprehends a rope, wire, chain, or any object which essentially functions in the same manner as such a cable.